1. Field of the Invention
The present invention relates to an input signal detector, and more particularly, to an input signal detecting apparatus and method for detecting whether or not an input signal exits in a digital television (TV).
2. Discussion of the Related Art
Recently, with conversion of an analog broadcast (such as a terrestrial broadcast and a cable broadcast) into a digital broadcast, a time required for scanning channel information is greatly increased as compared to the conventional analog broadcast. This is because 5 through 12 broadcasts exit in one channel and empty channels may exist in a broadcast band in case of the digital broadcast.
Additionally, in case of the cable broadcast, channel information is frequently changed according to a local broadcast station's condition, whereby channel information should be more frequently scanned.
A problem of the prior art at the time of the channel information scanning will now be described with reference to the accompanying drawings.
FIG. 1 is a block diagram of a vestigial sideband (VSB) receiver according to an advanced television systems committee (ATSC) standard of a general digital TV.
A VSB receiver shown in FIG. 1 is an example of a receiver that uses an automatic gain controller (AGC) for directly controlling gains of a radio frequency (RF) and an intermediate frequency (IF).
Referring to FIG. 1, the VSB receiver is constructed to include a tuner 1, an IF AGC amplifier 2 and a VSB demodulator (VSB Rx chip) 3. The tuner 1 tunes only a channel frequency selected by a user from RF signals received through an antenna, and then controls the tuned RF signal to thereby convert the tuned RF signal into an IF signal. The IF AGC amplifier 2 controls and amplifies a gain of the IF signal. The VSB demodulator 3 VSB-demodulates the gain-controlled IF signal, and outputs gain control signals (RF AGC control, IF AGC control) respectively to the tuner 1 and the IF AGC amplifier 2, and directly controls gains of the RF and IF signals. Here, the VSB demodulator 3 is equipped with an AGC 3-1 for generating an IF gain control signal (IF AGC control) and an RF gain control signal (RF AGC control).
An operation of the VSB receiver will now be described with reference to FIG. 1.
Firstly, when a VSB-modulated RF signal is received through the antenna, the tuner 1 selects a desired channel frequency by tuning and then converts a tuned RF signal into an IF signal to thereby output the IF signal to the IF AGC amplifier 2. The IF AGC amplifier controls a gain of the IF signal to thereby output the gain-controlled IF signal to the VSB demodulator 3. The VSB demodulator 3 VSB-demodulates the gain-controlled IF signal.
At this time, in order to maintain a constant signal gain, the AGC 3-1 of the VSB demodulator 3 compares the strength of the VSB-demodulated signal with a pre-stored gain information value. If a gain of the received signal is determined to be small from the result of the comparison, the VSB demodulator 3 generates a gain-up signal for increasing the received signal's gain. Otherwise, if a gain of the received signal is determined to be large from the result of the comparison, the VSB demodulator 3 generates a gain-down signal for decreasing the received signal's gain. The VSB demodulator 3 then outputs IF gain control signals (the gain-up signal and the gain-down signal) to the IF AGC amplifier 2.
The IF AGC amplifier 2 increases or decreases a gain of the IF signal according to the gain-up signal or the gain-down signal to thereby cause an IF signal to have a desired gain.
In an actual gain control method, after a gain is set to a maximum gain that the VSB receiver can have at the power-on time or at the time of the channel change, the tuner 1 controls a gain of the RF signal through the RF gain control signal in advance to the strength of an input signal.
If the strength of an output signal of the VSB demodulator 3 is not a desired strength even though a gain control capability of the tuner 1 reach the limit, the VSB demodulator 3 controls a gain of the IF AGC amplifier 2 through the IF gain control signal.
FIG. 2 is a graph illustrating an operational area and gain curves of the AGC 3-1 according to signal strength.
Referring to FIG. 2, the operational area is classified into a weak electric field area, a middle electric field area and a strong electric field area according to the strengths of input signals.
That is, an RF gain is maximally decreased and an IF gain is adjusted in the strong electric field area, whereby a saturation of an amplifier or a mixer is prevented.
Also, an IF gain is maintained at a proper magnitude and an RF gain is adjusted in the middle electric field area. An RF gain is maximally increased and an IF gain is adjusted in the weak electric field area.
In the weak/middle/strong electric field areas, an input range is classified into a noise range, a validity input range and a saturation range.
The validity input range indicates the Max/Min strengths of input signals that the VSB receiver can receive. The noise range indicates the strengths of input signals that the receiver cannot discriminate from a noise because the input signal is zero or very weak.
The saturation range means the strengths of input signals that exceed an input range of the receiver of the amplifier to thereby induce a saturation state because the input signal is too strong. In this case, the input signals are distorted, and the receiver cannot normally receive the distorted input signals.
The conventional VSB receiver having such a gain curve is diverged if a symbol recoverer, a carrier recoverer and an equalizer are operated even when an input signal is too weak for its symbol to be detected. On the contrary, when an input signal is too strong not to be saturated with respect to the input range, the input signal is distorted and the conventional VSB receiver cannot normally recover the distorted input signal.
In addition, during the channel information scanning, the conventional VSB receiver recovers a symbol, decodes transmitted data from the symbol, and then waits for a constant while until en error does not exist. Thereafter, the conventional VSB receiver checks whether or not a broadcast signal is generated to thereby obtain channel information. Accordingly, the conventional VSB receiver inefficiently waits for a constant while at all times even when no channel exists, to thereby greatly increasing a time required for scanning channel information. Actually, the channel information scanning requires several tens of minutes, thereby causing the user's inconvenience.